1. Field of the Invention
Embodiments of the invention relate to the field of semiconductor device fabrication. More particularly, the present invention relates to an apparatus for controlling the temperature of an RF window in an RF ion source thereby suppressing the formation of deposits thereon.
2. Discussion of Related Art
Ion implantation is a process used to dope ions into a work piece or target substrate. One type of ion implantation is used to implant impurity ions during the manufacture of semiconductor substrates to obtain desired electrical device characteristics. An ion implanter generally includes an ion source chamber which generates ions of a particular species, a series of beam line components to control the ion beam and a platen to secure the target substrate that receives the ion beam. These components are housed in a vacuum environment to prevent contamination and dispersion of the ion beam. The beam line components may include a series of electrodes to extract the ions from the source chamber, a mass analyzer configured with a particular magnetic field such that only the ions with a desired mass-to-charge ratio travel through the analyzer, and a corrector magnet to provide a ribbon beam which is directed to the platen orthogonally with respect to the ion beam to implant the ions into the target substrate. The ions lose energy when they collide with electrons and nuclei in the substrate and come to rest at a desired depth within the substrate based on the acceleration energy. The depth of implantation into the substrate is based on the ion implant energy and the mass of the ions generated in the source chamber. Typically, arsenic or phosphorus may be doped to form n-type regions in the substrate and boron, gallium or indium are doped to create p-type regions in the substrate.
Various types of ion sources may be employed based on the type of plasma desired as well as the associated beam profile for implantation in the target substrate. One type of ion source is a hot-cathode ion source that utilizes an indirectly heated cathode (IHC) as the heating element to ionize a feed gas introduced into the chamber to form charged ions and electrons (i.e. plasma). The feed gas includes element(s) to be implanted into the target substrate. Another type of ion source is an RF plasma source which utilizes an RF coil to excite a feed gas supplied to the chamber. The current in the RF coil can be adjusted to control the density of the generated ions extracted from the chamber.
An RF ion source can accommodate large-size ion beam extractions and typically has a longer operational life as compared to an IHC source since RF ion sources do not utilize a hot cathode element. However, RF ion sources are typically operated at relatively low temperatures and are limited for use with inert gases and fluorides. This is due to the fact that when hydrides are used in RF ion sources, the hydrides cause deposits therein. These deposits reduce the RF power coupling efficiency to the feed gas in the source chamber resulting in low plasma density, unstable discharge, glitching and source failures. Glitching is a sudden transient in the beam current that can adversely affect precise dose control and dose uniformity of the implanted species on a target substrate. This may cause unstable ion source operation and beam extraction thereby compromising the desired beam profile which negatively impacts manufacturing throughput.
For certain RF ion source applications, using fluorides rather than hydrides have associated disadvantages. For example, using fluorides as feed gases such as Boron Trifluoride (BF3) result in low fractionation of the desired ion species such as Boron (B) based on the composition of the supplied feed gas. Low fractionation compromises the production of the desired ion species at a given extraction condition from the source chamber. In contrast, using hydrides as a feed gas produces higher fractionation of the desired ion species. However, hydrides tend to leave deposit materials typically on cold surfaces. Suppression of these deposits when using hydride gases in an RF ion source may be accomplished by proper heating of the source chamber as well as the extraction electrodes used to extract the ion beam from the source. In particular, by raising the window temperature in an RF ion source, deposition can be suppressed and also easily removed from the RF window via ion bombardment. Thus, there is a need to provide a temperature controlled RF ion source that suppresses unwanted depositions for stable, glitch-free ion source operation.
In addition, electrostatic shielding (typically referred to as “Faraday Shield”) reduces the capacitive coupling of the high voltage RF antenna to the plasma within the chamber for RF ion sources. This is especially the case when the antenna is located outside of the RF window, in which capacitive coupling can cause RF window material sputtering and also poor coupling with the plasma. The electrostatic shield is typically an electrically conductive plate with multiple-shaped slots, located in between RF antenna and RF window. The electrostatic shield suppresses capacitive coupling of the RF power into the plasma, which is much less effective in plasma generation than inductive coupling. Instead, the electrostatic shield allows only inductive components to penetrate through the shielding thus accommodating the generation of high-density plasmas. However, previous RF ion sources, that include electrostatic shields are configured to only provide this shielding function. Thus, there is an additional need to provide an RF ion source wherein the heating elements may have the dual function of ion source heating and electrostatic shielding.